1. Field of the Invention
The present invention relates to a process for the removal of nitrogen contained in natural gas.
More specifically, the present invention relates to a process for the reduction of the nitrogen content in natural gas to a concentration of less than 10% molar.
2. Description of Related Art
As is known, natural gas has become a source of thermal energy which is one of the main alternatives to traditional fuels of a fossil nature, in particular fuel oils of petroleum origin, considered as being one of the main causes of the greenhouse effect which influences the climatic trend of the planet.
Natural gas, coming from production fields, essentially consists of methane but may also contain, in addition to significant traces of upper C2-C7+ hydrocarbons, varying quantities of inert gases, for example carbon dioxide or nitrogen, whose presence must be eliminated or reduced to satisfy specifications of use.
Among these specifications is that of respecting the Wobbe index, a parameter Refined by the ratio between the calorific value (high or low) of gas and its density with respect to air. The Wobbe index is therefore a parameter which represents a measurement of the heat which is produced by the gas when burnt at a constant pressure.
Methods for the removal of inert gases, in particular nitrogen, from natural gas, are known in scientific literature. Most of these processes, however, are essentially based on removing nitrogen cryogenically as described, for example, in U.S. Pat. Nos. 5,505,049, 5,036,671 or 4,415,345, with effective but not economic results.
U.S. Pat. No.5,321,952 discloses an alternative to cryogenic processes, which comprises absorbing the hydrocarbon fraction of natural gas (essentially methane) in a C9-C14 paraffinic oil and discharging the inert gases (essentially nitrogen) thus separated into the atmosphere or another operating unit. The use of paraffinic oil as absorbing liquid, however, implies a series of disadvantages which make the absorption process, as an alternative to cryogenic processes, much less competitive with respect to the latter.
Above all, the absorption process with paraffinic oil requires particular operating conditions. In fact, even if there is the possibility of operating at room temperature, in practice, it is advisable to operate at a temperature ranging from xe2x88x9240 to xe2x88x9210xc2x0 C. with the consequent necessity of forced dehydration of the gas to avoid freezing phenomena inside the equipment.
A second disadvantage, much more serious than the first, occurs in the desorption phase for the recovery of the gas. This operation takes place by expansion of the paraffinic oil in flash columns arranged in series. At the end of the expansion, the paraffinic oil is recycled to the absorption whereas the gas is, partly sent to a compression section to be fed to a distribution network, and is partly recycled to the absorption. This compression phase alone clearly makes the process less competitive.
A further disadvantage of the process described in U.S. Pat. No. 5,321,952 can be identified in the absorption section where it is necessary to operate with two columns, one fed with natural gas coming from the production, the other with recycled gas.
The Applicant has now found that the simple substitution of paraffinic oil with a lighter and less viscous liquid, for example, with a virgin naphtha, surprisingly eliminates the above drawbacks. At the same time, a separation process is obtained, which is as effective as cryogenic systems but without the high costs involved.
The object of the present invention therefore relates to a process for the removal of nitrogen contained in natural gas which comprises:
a) absorbing the hydrocarbon component of natural gas by means of virgin naphtha, essentially consisting of C5-C8 paraffins, in an absorption device, discharging the non-absorbed nitrogen;
b) stripping the hydrocarbon component from the virgin naphtha in a stripping column operating at a temperature at the bottom ranging from 150 to 200xc2x0 C.;
c) recycling the virgin naphtha, recovered in the stripping, to step (a);
d) feeding the stripped hydrocarbon component to a distribution network.